If you notice the M4 will be the common primary out in the field because they're everywhere and everyone makes them.. they're easy to diagnose when problems arise... and there's millions of tacticool accessories for them.

Your short arm will be holstered for a majority of games... I carry a pistol for the complete look for my load out. 90% of the people I game with don't even carry or have one.

Another primary fire arms to consider is a Cybergun Galil - 370FPS very quiet and with a Firefly hop up rubber... very accurate (thanks Amos! ) Very unique weapon and rarely seen in the field.